Spark plug with improved intensifier spark gap in the center electrode



June 27, 1967 P. N. KESTEN ETAL SPARK PLUG WITH IMPROVED INTENSIFIER SPARK ll 4 lkviorzrk Filed Oct. 23, 1965 GAP IN THE CENTER ELECTRODE faf United States Patent 3,328,621 SPARK PLUG WITH IMPROVED INTENSIFIER SPARK GAP IN THE CENTER ELECTRODE Patrick N. Kesten, Davison, and Karl Schwartzwalder,

Holly, Mich., assignors to General Motors Company,

Detroit, Micln, a corporation of Delaware Filed Oct. 23, 1965, Ser. No. 503,639 Claims. (Cl. 313-124) This invention relates to an improved spark plug of the type having an auxiliary spark gap incorporated in the center electrode.

It has long been known that an auxiliary spark gap in series with the firing gap of a spark plug can function to control the breakdown voltage at the firing gap and and enable the plug to function efiiciently despite accumulations of carbon or the like in the firing gap which would otherwise cause fouling. However, this advantage has not been attained to its fullest extent with prior auxiliary gap constructions principally because they have not been capable of providing the desired constancy of breakdown voltage over a long period of operation. The prior art auxiliary spark gaps are constructed of two layers of metal powder having interposed therebetween a compacted mass of alumina grain. This auxiliary spark gap is positioned in the insulator centerbore so that one layer of metal powder is in contact with the conductive glass seal which looks the head of the center electrode while the other layer of metal powder is in contact with the conductive glass seal which locks the end of the terminal screw. The breakdown voltage of this type prior art auxiliary gap construction varies primarily because the aluv mina grain in the gap area separating the two layers of metal powder does not have sufiicient air to function properly for the life of the spark plug. The alumina gap area does not have sufiicient air to function properly because the conductive glass seal surrounding the end of the terminal screw and the layer of metal powder adjacent to this glass seal have hermetically sealed the alumina gap area. The hermetically sealed alumina gap area will function properly until the air in the alumina spark gap area is used up. After the air is used up, the gap breakdown voltage does not remain constant.

Another disadvantage of the prior art auxiliary gap constructions is the decrease in the electrical resistance of the spark gap which is sufi'icient to cause the gap to (be inoperative. The lowering of the electrical resistance of the auxiliary spark gap sufliciently to cause the gap to be inoperative is due to a conductive carbon film which is deposited in the alumina spark gap area. This carbon film is deposited during the sealing operation when the organic binder in the glass powder mixtures is thermally degraded and vaporized. This invention describes a spark plug with an auxiliary spark gap which is an improvement over that described in U.S. Patent 2,988,662, granted June 13, 1961 to Smith.

It is a basic object of this invention to provide an improved spark plug With an auxiliary gap construction which retains constant electrical characteristics for the life of the spark plug.

This and other objects are accomplished in accordance with the invention by an auxiliary spark gap construction located in the insulator centerbore consisting of one layer of a conductive heat-resistant metal powder and one air permeable layer of conductive heat-resistant metal powder which serve as electrodes having interposed therebetween a compacted mass of alumina powder which serves as an auxiliary spark gap. The auxiliary gap electrodes are sandwiched between a hermetic conductive glass seal which surrounds the center electrode head and an air permeable conductive layer of crystallized glass which surrounds the lower end of the terminal screw. Both the glass seal and the crystallized glass layer are formed from metal-glass powder mixtures which contain a carbon-free inorganic binder which does not give off carbon vapors during the sealing operation. This construction provides a carbon-free auxiliary spark gap which can breathe air for the life of the spark plug thereby enabling the auxiliary gap to retain constant electrical characteristics for the life of the spark plug.

Other objects and advantages of this invention will be apparent from the following detailed description, reference being made to the accompanying drawings wherein a preferred embodiment of this invention is shown.

Referring now to the embodiment shown in the drawing, the spark plug 10 comprises a conventional outer metal shell 12 having a ground electrode 14 welded to the lower end thereof. An insulator -16 is secured within the shell 12 in the conventional manner. The insulator 16 is formed with a centerbore having a lower portion 18 of relatively small diameter and an upper portion 20 of larger diameter. Positioned in the lower portion 18 of the insulator centerbore is the center electrode 22, the lower end thereof projecting beyond the lower tip of the insulator 16 into spaced relationship with ground electrode 14 to form the firing gap of the spark plug- The center electrode head 24 rests on the insulator centerbore ledge 25, the insulator centerbore ledge 25 connecting the lower portion 18 to the upper portion 20 of the insulator centerbore. A lower conductive metal glass seal 26 surrounding the center electrode head 24 forms a hermetic seal in the insulator centerbore portion 20.

The composition of the metal-glass powder mixture which forms the metal glass seal 26 must form a hermetic seal without giving off any carbon vapor when the seal is formed. An example of a metal glass powder mixture which may be advantageously employed for the lower metal glass seal 26 consists of 45 parts by weight glass which will be hereinafter described, 20 parts by weight copper powder, 35 parts by weight nickel powder and 1 part by weight bentonite. Bentonite, an inorganic binder, is used since it does not give off any carbon which would form a conductive film over the spark gap material alumina during the formation of the seal. Bentonite is a clay containing appreciable amounts of the clay mineral montmorillonite which is principally aluminum and silicate, usually with some magnesium and iron. Other clays may be used as the inorganic binder in this metal-glass powder mixture. The mesh size of the clay binder is not critical, fine mesh size of the order of mesh or finer, however, are easier to mix. The concentration of the inorganic binder in the metal-glass powder mixture is from 0.5 to 3.0 weight percent.

The metal powder in metal glass seal 26 must not be oxidized during the formation of the seal, since oxidation of the metal powder will change the electrical resistance in the seal as well as preventing the glass phase from sealing properly. Metal in a metal glass powder mixture containing an organic binder is not oxidized during the formation of the seal because the decomposed organic binder forms a reducing atmosphere of carbon vapor which prevents oxidation. A carbon-free inorganic binder such as bentonite, which is used in this invention to avoid depositing carbon on the spark gap alumina layer 32 hereinafter described in greater detail, does not provide a reducing carbon atmosphere and, as a result, metals such as copper will oxidize. Nickel powder, although it oxidizes to some extent, does not affect the conductivity of the seal. It was also observed that the copper powder in the nickel powder-copper powder mixture used in the example Will not oxidize. We are unable to explain why the presence of nickel powder will prevent oxidation of the copper powder. Nickel powder is the only powder of which we are aware which will prevent the copper powder from being oxidized. It is advantageous to use a mixture of copper and nickel powder instead of nickel powder alone since the copper powder is less expensive. The concentration of the metal powder in the metal glass mixture is from 50% by weight to 70% by weight. The concentration of the nickel powder in the copper-nickel powder mixture is from 60% by weight to 100% by weight. That is, nickel powder can be used alone but at least 60% of the copper-nickel metal powder mixture must be nickel.

The glass used in this seal is a typical glass used in spark plug glass seals, and consists of 65 weight percent SiO 23 weight percent B weight percent A1 0 and 7 weight percent Na O. The mesh size of the glass powder does not appreciably affect the performance of the seal. A mesh size of 200 for the glass powder is satisfactory. The glass concentration in the metal glass powder mixture is from 30 to 50 weight percent.

Above the glass seal 26 in the insulator centerbore portion 20 the auxiliary gap is formed, in accordance with the invention, by layers 28 and 30 of metal powder, having interposed therebetween the layer 32 of compacted alumina grain. The metal powder layer 28 is in intimate contact with the hermetic conductive metal glass seal 26 and serves as one of the electrodes of the auxiliary gap. The metal layer 28 should be a conductive heat-resistant metal, such as copper, nickel, tungsten, iron, silver, and the like. The particle size of the metal in this layer 28 should be such that the sparking in the auxiliary gap will not redeposit the metal powder in the alumina layer 32. Spherical nickel powder in which 54% of the particles would not pass through a 100 mesh screen sieve is used in metal powder layer 28 in the preferred embodiment. Compacted alumina grain 32 having a particle size of 60 mesh is preferred in the gap between the metal powder auxiliary gap electrodes since this particle size will not pack. Packing of the alumina is undesirable since this could cause a seal to be formed which would adversely affect the sparking in the auxiliary gap.

The metal powder layer 30 serves as the second electrode 0f the auxiliary gap and is in intimate contact with the conductive air permeable crystallized glass layer 34 which is positioned above the layer 30. This metal powder layer 30 must be air permeable in order to permit the auxiliary spark gap area to breathe air. As was indicated earlier, it is essential in providing a constant breakdown voltage over a long period of operation that the auxiliary spark gap be able to breathe air. It is necessary in forming an air permeable metal powder layer 30 that there be a sufiicient number of coarse particles, that is particles which do not pass through a 100 mesh screen sieve. Commercially available spherical nickel powder is used in the preferred embodiment. As mentioned earlier, the particle size of commercially available spherical nickel powder is such that 54% of the particles are too large to pass through a 100 mesh screen. The coarse metal powders in powder layer 30 enable this electrode to be air permeable as well as preventing auxiliary gap sparking from redepositing the metal powder in the alumina layer 32. The coarse particles also prevent a seal from forming on the interface between the metal layer 30 and the glass layer 34 which has been observed to take place when the powder does not contain any coarse particles larger than 100 mesh.

Coarse particles which do not pass through a 100 mesh screen sieve appear to be required in layer 30 to insure an auxiliary spark gap life of over 10,000 miles in field tests. Spark plugs having an auxiliary spark gap of the type described in this invention wherein commercially available spherical nickel powder having a sufficient number of coarse particles was used in metal powder layer 30 performed satisfactorily without spark plug failure in field tests in which cars were driven for 10,000 miles. Spark plugs having a particle size of 200 mesh and finer in the upper metal layer 30 did not perform satisfactorily in this field test since some spark plug failures were noted after 2,000 miles and more failures were observed before the completion of the 10,000 mile test. It is to be noted that auxiliary spark gaps in which the metal layer 30 contains metal particles 200 mesh and finer will function satisfactorily for a period of time which is believed to be dependent upon the air permeability of the metal layer 30 and the glass layer 34.

The crystallized glass layer 34 forms a seal around the terminal screw 36 and locks it in the insulator centerbore above the metal powder layer 30. The glass layer 34 must be air permeable in order to permit the auxiliary spark gap area to breathe air. An example of a conductive air permeable glass layer contains 69% of a crystallized glass as will be hereinafter described, 29% by weight copper powder, 2% by weight boric anhydride, and 2% by weight bentonite. Bentonite, an inorganic binder, is used since it does not give off any carbon which would form a conductive film in the spark gap material alumina during the formation of the seal. The concentration of the inorganic binder in the metal glass powder mixture is from 0.5 to 3.0 weight percent. The crystallized glass used in glass layer 34 is a type of glass which forms a porous structure. An example of a crystallized glass is commercially avail-able Ferro glass FB-l62M consisting of 60 weight percent SiO 30% by weight PhD, 1% by weight A1 0 4% by weight TiO 2% by weight Na O, and 1% by weight BaO. Crystallized glasses having porous structures may be made by adding 1 to 25% by weight titania (TiO or by adding 1 to 25% by weight zirconia (ZrO to glass frits, the actual percentages depending upon the other ingredients in the glass frit. The concentration of the crystallized glass in the glass layer 34 is from 65% by weight to by weight. Concentrations below this range are not air permeable enough due to the presence of the metal powder. Concentrations of crystallized glass above 85 do not have sufiicient conductivity. Boric an hydride is added to enhance the air permeability of the glass, since boric anhydride has a high coefiicient of expansion. The concentration of the boric anhydride is from 0.5 to 3% by weight. Any conductive heat-resistant metal powder may be used in this glass layer 34, such as copper, nickel, tungsten, iron, silver, and the like. The concentration of the metal in glass layer 34 is from 14% to 30% by weight. When glass layer 34 contains more than 30% metal powder the glass layer lacks air permeability, whereas when the concentration is below 14%, the glass layer does not have sufiicient conductivity. The conductivity of the glass layer does not have to be as high as the conductive metal glass seal 26 because the end 38 of the terminal screw 36 is in close proximity to the metal powder layer 30. Since the quantity of glass layer 34 separating the end 38 from the metal powder layer 30 is small, the amount of metal required for conductivity purposes is not as large. For that same reason, it is not necessary to protect the copper metal powder from being oxidized in glass layer 34, since the conductance of glass layer 34 is not as important as in the lower seal 26.

The auxiliary gap of this invention may be formed in accordance with the method described in United States Patent 2,988,662, referred to earlier.

It is to be understood that although the invention has been described with specific reference to a particular em bodiment thereof, it is not to be so limited since changes and alterations therein may be made which are within the full and intended scope of the claims which follow.

What is claimed is:

1. A spark plug comprising a metal shell having a ground electrode secured to one end thereof, an insulator having a centerbore therethrough secured within said shell, a center electrode positioned in one end of said centerbore having a head portion formed at the upper end thereof and a lower portion extending through said insulator into spaced relationship with said ground electrode to form a spark gap therebetween, a conductive sealing layer in said centerbore positioned above and about the head portion of said center electrode and bonded to the insulator walls, a compacted layer of high heat and erosionresistant metal powder in said centerbore which serves as one of the auxiliary gap electrodes positioned above and in contact with said conductive sealing layer, a compacted layer of carbon-free alumina grain in said centerbore positioned above and in contact with said metal powder layer, a compacted air permeable layer of coarse high heat and erosion-resistant metal powder in said centerbore which serves as the other of the auxiliary gap electrodes positioned above and in contact with said alumina layer, said alumina layer serving as an auxiliary spark gap between said metal powder layers, a conductive air permeable layer of crystallized glass in said centerbore positioned above and in contact with said air permeable metal powder layer, said air permeable layer of crystallized glass in combination with said air permeable layer of metal powder forming a passageway to admit a sufiicient amount of air to said auxiliary spark gap layer to maintain constant electrical characteristics for the life of the spark plug, and a metal contact element in said centerbore having a lower portion embedded in said conductive air permeable layer of crystallized glass.

2. A spark plug as defined in claim 1 wherein said conductive sealing layer has been formed from a carbonfree metal glass powder mixture.

3. A spark plug as defined in claim 1 wherein said conductive sealing layer contains a copper-nickel powder mixture in which said mixture contains to by weight nickel.

4. A spark plug as defined in claim 1 wherein said conductive sealing layer contains an inorganic binder.

5. A spark plug as defined in claim 1 wherein said air permeable layer of coarse, high heat and erosion-resistant metal powder contains metal powder particles which will not pass through a 100 mesh screen sieve.

6. A spark plug as defined in claim 1 wherein said air permeable layer of coarse, high heat and erosion-resistant metal powder is spherical nickel.

7. A spark plug as defined in claim 1 wherein said layer of crystallized glass has been formed from a carbonfree metal glass powder mixture.

8. A spark plug as defined in claim 1 wherein said crystallized glass layer contains between 14 and 30% by weight metal powder.

9. A spark plug as defined in claim 1 wherein said crystallized glass layer contains titania.

10. A spark plug as defined in claim 1 wherein said crystallized glass layer contains zirconia.

No references cited.

JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. A SPARK PLUG COMPRISING A METAL SHELL HAVING A GROUND ELECTRODE SECURED TO ONE END THEREOF, AN INSULATOR HAVING A CENTERBORE THERETHROUGH SECURED WITHIN SAID SHELL, A CENTER ELECTRODE POSITIONED IN ONE END OF SAID CENTERBORE HAVING A HEAD PORTION FORMED AT THE UPPER END THEREOF AND A LOWER PORTION EXTENDING THROUGH SAID INSULATOR INTO SPACED RELATIONSHIP WITH SAID GROUND ELECTRODE TO FORM A SPARG GAP THEREBETWEEN, A CONDUCTIVE SEALING LAYER IN SAID CENTERBORE POSITIONED ABOVE AND ABOUT THE HEAD PORTION OF SAID CENTER ELECTRODE AND BONDED TO THE INSULATOR WALLS, A COMPACTED LAYER OF HIGH HEAT AND EROSIONRESISTANT METAL POWDER IN SAID CENTERBORE WHICH SERVES AS ONE OF THE AUXILIARY GAP ELECTRODES POSITIONED ABOVE AND IN CONTACT WITH SAID CONDUCTIVE SEALING LAYER, A COMPACTED LAYER OF CARBON-FREE ALUMINA GRAIN IN SAID CENTERBORE POSITIONED ABOVE AND IN CONTACT WITH SAID METAL POWDER LAYER, A COMPACTED AIR PERMEABLE LAYER OF COARSE HIGH HEAT AND EROSION-RESISTANT METAL POWDER IN SAID CENTERBORE WHICH SERVES AS THE OTHER OF THE AUXILIARY GAP ELECTRODES POSTIONED ABOVE AND IN CONTACT WITH SAID ALUMINA LAYER, SAID ALUMINA LAYER SERVING AS AN AUXILIARY SPARK GAP BETWEEN SAID METAL POWDER LAYERS, A CONDUCTIVE AIR PERMEABLE LAYER OF CRYSTALLIZED GLASS IN SAID CENTERBORE POSITIONED ABOVE AND IN CONTACT WITH SAID AIR PERMEABLE METAL POWDER LAYER, SAID AIR PERMEABLE LAYER OF CRYSTALLIZED GLASS IN COMBINATION WITH SAID PERMEABLE LAYER OF METAL POWDER FORMING A PASSAGEWAY TO ADMIT A SUFFICIENT AMOUNT OF AIR TO SAID AUXILIARY SPARK GAP LAYER TO MAINTAIN CONSTANT ELECTRICAL CHARACTERISTICS FOR THE LIFE OF THE SPARK PLUG, AND A METAL CONTACT ELEMENT IN SAID CENTERBORE HAVING A LOWER PORTION EMBEDDED IN SAID CONDUCTIVE AIR PERMEABLE LAYER OF CRYSTALLIZED GLASS. 